Turbo Compounding Is the Next Big Thing in Energy Recovery

Internal combustion will play a role in automotive propulsion far into the future, but not without help. At issue is the 25- to 30-percent energy loss that occurs when hot exhaust gases are dumped into the atmosphere.

Turbochargers successfully recycle some of that energy, but there are other recovery technologies carmakers haven't tapped. One that's especially promising is turbo compounding.

Like a turbocharger, the turbo-compound approach recovers waste-exhaust energy, but, instead of powering a compressor, the turbine wheel is connected to the crankshaft through a train of gears. Turbo compounding works with or without a conventional turbocharger upstream.

Just after World War II, the Wright R-3350 18-cylinder aircraft engine used three turbines to recover 20 percent of the exhaust energy in Douglas DC-7 and ­Lockheed Super Constellation airliners. Freightliner and Western Star semis powered by Detroit Diesel DD15 TC engines employ this technology today.

To avoid the complexity and cost of turbine-to-crankshaft gearing, the turbo-compound approach alternatively can be used to drive a generator. Such a device is used in Formula 1 where it's called a "motor/generator unit-heat." Here the exhaust turbine, compressor wheel, and generator are all mounted on a common shaft. The recovered electrical energy powers the wheels out of a turn (through a second device called "motor/generator unit-kinetic"). The generator can also operate as a motor to preemptively spin the compressor to eliminate boost lag.

Production cars could use the electrical energy recovered by turbo compounding to eliminate engine-driven alternators. In hybrid applications, turbo compounding is an efficient way to charge batteries and to power electric-drive motors.

With electric assist expected to become commonplace in gas-powered cars, turbo compounding might be the technology that further solidifies automakers' current affinity for exhaust-driven turbines.

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